Dc to ac inverter with single-switch bipolar boost circuit

ABSTRACT

This invention improves the performance and lowers the cost of DC to AC inverters and the systems where these inverters are used. The performance enhancements are most valuable in renewable and distributed energy applications where high power conversion efficiencies are critical. The invention allows a variety of DC sources to provide power thru the inverter to the utility grid or directly to loads without a transformer and at very high power conversion efficiencies. The enabling technology is a novel boost converter stage that regulates the voltage for a following DC to AC converter stage and uses a single semiconductor switching device. The AC inverter output configuration is either single-phase or three-phase.

BACKGROUND OF INVENTION

[0001] Photovoltaic cells produce DC power over a wide voltage rangedepending on the amount of sunlight, ambient temperature and wind speed.A minimum DC voltage is required to directly convert this DC voltage toa standard 120 Volts AC and to do so without the use of a 60 cycletransformer. There are National Electric Code restrictions andclass-of-equipment considerations that make photovoltaic arrays muchmore cost effective when sized for a maximum of 600 Vdc. The problem isthat under some conditions, photovoltaic arrays sized for this 600 Vdcmaximum will not meet the said minimum voltage requirements for directDC to AC conversion. The prior art inverters would either use a 60 cycletransformer, a dual boost converter input stage or a full-bridge inputstage with a high frequency transformer to achieve the proper voltagematch over the predicted range of inverter operation. A 60 cycletransformer decreases power conversion efficiency and adds to theoverall inverter or system costs. A dual boost converter input stage ora full-bridge input stage adds complexity to and lowers the conversionefficiency of the inverter. The prior art, dual boost converter and fulldirectly or indirectly, is old technology and is well known. Thesingle-switch bipolar boost converter, disclosed herein, is a novelreplacement for the dual boost converter. The single-switch bipolarboost converter is less complex, lower cost and provides higher powerconversion efficiencies.

DETAILED DESCRIPTION

[0002] The invention is more related to the power circuit topology of aninverter than the control methods. The inverter topology is novel whilethe control methods are known. The preferred embodiment of the inventionis shown in FIG. 1 and is illustrated as part of a system consisting ofthree components; an inverter 70, a photovoltaic array 30 and a typical120/240 Vac, split-phase, residential, electric utility service 60. Theinverter 70 is the embodiment of the invention and is further brokendown into to two functional blocks, the boost converter 40 and the DC toAC converter 50. The photovoltaic array 30 and the electric utilityservice 60 serve to illustrate the use and usefulness of the invention.The system described converts solar energy to electric power andfunctions as a distributed generator on the electric utility grid. Formaximum power conversion efficiency, it is desirable to regulate aconstant voltage on capacitor 12 slightly higher than the peak voltageson the utility grid 60. Boost converter 40 performs this function. Whenthe series voltage of photovoltaic arrays 2 and 3 is higher than thevoltage on capacitor 12, current flows into capacitor 12. If a highervoltage on capacitor 12 is desired, Insulated Gate Bipolar Transistor(IGBT) 9 is closed, charging inductors 6 and 7 and back biasing diodes 10 and 11. When IGBT 9, is opened the stored energy in inductors 6 and 7is delivered to capacitor 12. The duty cycle or on/off time ratio ofIGBT 9 is proportional to the ratio of regulated voltage on capacitor 12and the series voltage of photovoltaic arrays 2 and 3. The frequency ofoperation is typically upwards of 20 kHz. The circuit controlling IGBT 9uses the voltage sensed across capacitor 12 and the current sensed withcurrent sensor 8. The closed loop regulation method is known includingalgorithms for tracking the maximum power point of the photovoltaicarray. For clarity, the control circuit interface is not shown.Capacitors 4 and 5 shunt high frequency currents to ground. The DC to ACconverter 50 is a known H-bridge configuration with IGBT switches 13,14, 18, 19 and freewheeling diodes 15, 16, 20, 21. The Pulse WidthModulated (PWM) sinusoidal current regulation method for utilityinteractive inverters is known. Inductor 22 and capacitor 23 form a2-pole filter that removes high frequency PWM components, as do inductor24 and capacitor 25. The control circuit uses current sensor 17 toregulate sinusoidal current into the utility grid, synchronized with theutility grid voltage for unity power factor power transfer. The controlcircuit also uses current sensor 17 to precisely regulate DC currentcomponents to near zero. These control algorithms are known. If inductor7 and diode 11 were replaced by short circuits, the typical, known,monopole boost circuit configuration is had. An inverter so configuredcould not be used with a grounded photovoltaic array unless a 60 cycleisolation transformer was used at the utility interface. This sameinverter used with a floating, non-grounded photovoltaic array could beused without a transformer but undesirable, common-mode, 60 cycle andhigh frequency voltage components would be imposed on the array withrespect to ground. With the inclusion of inductor 7 and diode 11, asingle semiconductor switch can generate a bipolar voltage with respectto ground, enabling a system configuration with no transformer and withno common mode array voltage with respect to ground. This inventionfacilitates high power, high frequency, lower cost DC to AC powerconversion over a wide DC input range with a minimum number ofsemiconductor switches. This invention also facilitates an inverter thatis intrinsically low in Electromagnetic Interference (EMI) productionbecause each ungrounded input and output terminal is connected inparallel with a capacitor and in series with an inductor.

1. An inverter for utility interactive applications for converting DCpower from solar photovoltaic modules into AC power and comprising: a DCto DC boost converter that generates a bipolar voltage with respect toground and uses a single semiconductor switch; a DC to AC converterwhich converts the output from said DC to DC converter into currentregulated sine waves, synchronized with the utility voltage, to sourcepower into the utility grid.
 2. A system for generating AC power from DCsources comprising: a DC source; a DC to DC converter for regulatingsaid source, wherein said DC to DC converter generates a bipolar voltagewith respect to ground and uses a single semiconductor switch and a DCto AC converter which converts the output from said DC to DC converterinto regulated current or voltage sine waves.
 3. An inverter accordingto claim 1 wherein the switching devices are insulated gate bipolartransistors.
 4. An inverter according to claim 1 wherein the switchingdevices are field effect transistors.
 5. An inverter according to claim1 wherein the switching devices are a combination of insulated gatebipolar transistors and field effect transistors.
 6. An inverteraccording to claim 1 wherein the DC to AC converter section isconfigured for split-phase operation using two half-bridge circuits. 7.An inverter according to claim 1 wherein the DC to AC converter sectionis configured for single-phase operation using one half-bridge circuitand wherein the capacitive energy storage element at the output of theDC to DC converter and the input of the DC to AC converter is configuredwith two series capacitors where the common point is connected, directlyor indirectly, to the AC system neutral conductor.
 8. An inverteraccording to claim 1 wherein the DC to AC converter section isconfigured for three-phase operation using three half-bridge circuitsand wherein the capacitive energy storage element at the output of theDC to DC converter and the input of the DC to AC converter is configuredwith two series capacitors where the common point is connected, directlyor indirectly, to the AC system neutral conductor.
 9. A system accordingto claim 2 wherein the DC source has a grounded center tap.
 10. A systemaccording to claim 2 wherein the DC source is not grounded.
 11. Aninverter according to claim 1 wherein the intended DC source is a fuelcell, battery, genset, wind turbine or micro-turbine.
 12. An systemaccording to claim 2 wherein the DC source is a fuel cell, battery,genset, wind turbine or micro-turbine.
 13. A system according to claim 2wherein the input to the DC to DC converter has a grounded center tap.14. A system according to claim 2 wherein the input to the DC to DCconverter is not grounded.
 15. An inverter according to claim 1 whereinthe AC output is designed to support loads directly.
 16. A systemaccording to claim 2 wherein the inverter output supports loadsdirectly.